1. Field of the Invention
This invention relates to bicycle crankshafts, to ways of mounting sprockets onto such crankshafts, and to ways of installing such crankshafts in bicycle frames.
2. Description of the Prior Art
Many of today's bicycles are subjected to severe useage on dirt tracks and on mountain trails, creating greater need for strength and rigidity in their crankshafts. At the same time, lightness is also much needed and sought after. Crankshafts which are light, rigid, and strong, are therefore much in demand.
Current bicycle crankshafts are usually of two types; 1-piece cranks, and 3-piece cranks.
One-piece cranks have a zee-shaped crank wherein both arms are integral with a central spindle. The zee-shaped crank is partly inserted thorugh the bicycle's crank tube, and bearings are then installed to rotatably support the crankshaft. one-piece crankshafts have several drawbacks.
(a) Usually, solid steel forgings are used for 1-piece cranks, making them relatively heavy for their strength and rigidity. PA1 (a) Usually, the crank arms are solid aluminum forgings, which are heavy for their strength and rigidity. PA1 (b) The area of the four-sided taper is one of high stress for both the steel spindle and for the aluminum crank arms. Aluminum arms occasionally enlarge at that point. If a steel spindle is ever to break, it will break at the taper first, in most cases. PA1 (a) Loose spacers, or washers can be inserted between the crank arms and the bearings in the frame, allowing for easy lateral adjustment of the sprocket, to obtain correct chain alignment. PA1 (b) Because the arms can be made of formed and welded steel, they can be very thin and have large cross sections, making for a very efficient design in terms of weight, strength, and rigidity. PA1 (c) The welded joint between one of the crank arms and the spindle saves the weight of a separate boss, and is inexpensive to produce, compared to tapered or splined connections, and it transmits the loads to the spindle in an efficient way. PA1 (d) Larger and stronger 9/16 pedal threads can be used. PA1 (e) It is very easy to remove the entire crankshaft to change a sprocket; only a pinch bolt at the spline connection needs be loosened for the arm to come off. The spindle slides out, and the bolt holding the sprocket onto the pad is removed, freeing the sprocket. Sprocket changes can be made so quickly and easily as to eliminate the need for separate spacers and chainrings, such as those used with three-piece and one-piece cranks. PA1 1. To provide a tubular, two-piece crankshaft for bicycles which is light, strong, and rigid. PA1 2. To provide a bicycle crankshaft in which one arm is splined to the spindle and which can be removed by loosening only one pinch bolt and sliding the arm off the spline. PA1 3. To provide a bicycle crankshaft in which the spindle can be inserted through its support bearings, withdrawn from them, without making bearing adjustments. PA1 4. To provide a bicycle crankshaft which uses one bolt to secure the bicycle sprocket to the crank arm. PA1 5. To provide a bicycle crank arm which uses arms and a spindle having relatively large cross sections and thin walls, allowing for easy welding of one member to another, allowing for successful heat-treatment of the entire weldment, and providing a structure having the most strength and rigidy for its weight. PA1 6. To provide a bicycle crankshaft in which the spread between crank arms can be adjusted by adding or withdrawing spacers.
Where one-piece cranks have been produced out of tubular members which have been welded together, light, but relatively flexible and weak cranks have resulted, partly because of the requirement that one end of the crank must insert through the crank tube of the frame, which creates geometrical restrictions on the shape of such one-piece cranks, resulting in weak and flexible cranks, for their weight.
Three-piece crankshafts have a separate spindle, which is rotatably supported by bearings mounted in the crank tube of the bicycle frame. Crank arms are usually secured to the ends of the spindle by means of mating, four sided tapers, although cross pins or splined connections are sometimes used.
Such three-piece construction has several disadvantages:
Occasionally, tubular steel crank arms have been produced having a boss with an internal four-sided taper. Here, a heavy boss portion is required, and also, the steel-to-steel joint tends to come loose more frequently than does the usual aluminum-to-steel joint.
Three-piece cranks having two splined connections have been used with some success, although they are unduly heavy and require extra machining and spline orientation.
Neither one-piece cranks nor 3-piece cranks have been entirely satisfactory.
In both three-piece and one-piece crankshafts, sprocket changing is time consuming, because of the need to remove one of the crank arms, or to disassemble the crankshaft and its support bearings. To speed up such sprocket changes, spiders are fitted to the crankshafts which mount several different sizes of sprockets, thus allowing for quick sprocket changing without dismantling the crankshaft installation itself.
While this accomplishes the desired result, it would be more convenient to be able to remove the crankshaft with little difficulty and to be able to change sprockets quickly, without the need for separate spiders with their machined indexing diameters and bolting patterns.